Microbiological test methods for the determination of antibacterial compounds, particularly residues of antibiotics such as cephalosporin, penicillin, tetracycline and derivatives thereof and chemotherapeutics such as sulfa's, in fluids such as milk, meat juice, serum and urine have been known for a long time. Examples of such tests have been described in CA 2056581, DE 3613794, EP 0005891, EP 0285792, EP 0611001, GB A 1467439 and U.S. Pat. No. 4,946,777. These descriptions all deal with ready to use tests that make use of a microorganism and will give a result by the change indicated by an indicator molecule added to the test system. The principle is that when an antibacterial compound is present in the fluid in a concentration sufficient to inhibit the growth of the microorganism the color of the indicator will stay the same, while, when no inhibition occurs, the growth of the microorganism is accompanied by the formation of acid or reduced metabolites or other phenomena that will induce an indicator signal.
The test systems mentioned above include a test medium, such as an agar medium, inoculated with a microorganism, preferably a strain of Bacillus, Escherichia coli or Streptococcus, and a pH indicator and/or a redox indicator. The microorganism and the indicator are introduced into an optionally buffered agar solution, optionally nutrients are added to the solution and optionally substances to change the sensitivity to certain antimicrobial compounds are added to the solution. Finally the agar solution is allowed to solidify to form the test medium such that the microorganisms stay alive but cannot multiply because of lack of nutrients and/or low temperature. A suitable test should have the desired sensitivity with regard to the compounds to be tested for.
The problem with the test systems currently distributed on the market and/or described in literature is that they have a limited sensitivity towards certain antibiotics. One of the consequences of this problem is that for certain applications, for instance when threshold requirements are changed, an adequate test system cannot be made available with the current technology. There is thus a need for an improved test method that does not have this problem.